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= Primnoa resedaeformis = From Wikipedia, the free encyclopedia

Primnoa resedaeformis is a species of deep-sea soft corals of the order Alcyonacea. It is a species commonly known as ‘popcorn coral’.

Taxonomy
Primnoa resedaeformis is a deep-sea coral from the order of Primnoidae. Primnoidae corals are heterotrophic and reside below the photic zone. Deep-sea corals do not require sunlight to survive because they lack the symbiotic photosynthetic algae called zooxanthellae seen in other coral species. Soft corals belonging to the order Alcyonacea, such as P. resedaeformis, are called gorgonian corals and lack the rigid CaCO3 skeletons typical of hard corals. Instead, they have small spiny elements that are more flexible, hence the name soft corals.

Anatomy
Popcorn corals are asymmetrical and attaches to rigid substrates in cold deep water. While P. resedaeformis seems to be a singular organism, it consists of a colony of multiple individual polyps. Popcorn corals can grow up to 2 m tall and 7 m wide, and individual polyps have been recorded as 4 mm wide and 8-10 mm tall. P. resedaeformis can be distinguished from its relative, Primnoa pacifica, by fewer polyps with two main rows of large abaxial scales between the marginal and basal scales.

Large-Scale Distribution
Primnoa resedaeformis can be in the Western Atlantic from Virginia Beach to Greenland and from Scotland to Greenland in the Eastern Atlantic. Popcorn corals are a deepwater coral found at depths of 90-1000 m. Studies suggest a higher abundance of the corals are found in submarine canyons along the Western Atlantic.

Fine-Scale Distribution
The soft coral attaches to a rigid substrate and requires strong currents to deliver the required food and limit sediment build-up. Studies suggest niche segregation between soft corals in deepwater, which is seen by the increased abundance of popcorn corals found on the up-current side of boulders in the presence of other soft corals. The niche distribution suggests that popcorn corals do not rely on water flow directly towards the mouth. Instead, particle capture can occur in currents coming from the side or below the tentacle crown of the polyp.

Diet
Primnoa resedaeformis do not have photosynthetic zooxanthellae symbionts seen in other corals. They maintain a plankton and organic matter diet and obtain it by filter-feeding and collecting food from the water column. Due to its presence on the lower sides of boulders and rigid substrates, studies suggest that popcorn corals rely on the resuspension of organic matter as a feeding adaptation to habitats shared with other soft corals.

Trophic Interactions
Corals tend to be a common habitat of multiple organisms in shallow waters. It also functions as a habitat for organisms in the deep water, as deep-sea ecosystems are few and far between. Sea stars, snails, and other animals feed directly on Primnoidae corals, including P. resedaeformis. While being a food source for some animals, P. resedaeformis also provides shelter to small organisms. Since P. resedaeformis requires water flow to filter feed, other filter-feeding animals take advantage of this opportunity to feed in better conditions by physically attaching to P. resedaeformis. Crinoids, sea anemones, sponges, and basket stars are known to attach to dead arms of P. resedaeformis for this reason. Some animals use the skeleton of these soft corals for shelter, such as rockfish. Rockfish, shrimps, and crabs use P. resedaeformis as shelter by hiding amongst the segments. Studies show that 85% of rockfish use P. resedaeformis for protection.

Growth and reproduction
The growth rates of popcorn coral has not been well studied. P. resedaeformis form growth rings annually in North Atlantic environments, so an estimate of the age occurs by counting the annual growth rings viewed in cross-sections. Researchers found popcorn corals up to 700 years old by dating the age using Carbon-14 dating. Studies have shown that radial growth rates are between 0.0083-0.0215 cm and an average axial growth rate of 1.7 cm. Studies suggest that a high water velocity is a limiting growth factor for the height of other gorgonian corals. However, no factors have been found to limit the growth of P. resedaeformis colonies, and they have been found to reach heights of 1 m.

Reproduction in P. resedaeformis is not well studied; however, they undergo sexual reproduction as a dispersal mechanism. P. resedaeformis is one of a few soft corals that undergo gonochoric broadcast spawning, where gametes are released from both male and female colonies and meet in the water column. The number of oocytes formed by colonies is depth-dependent; corals in shallow water have been found to produce twice as many oocytes per polyp as deepwater colonies. Deeper water polyp colonies produce planktonic planula larvae that disperse in the water column. The planula larvae feed and disperse before they settle onto a suitable substrate. Recent studies found high recruitment, the transition from planktonic to sessile form, of P. resedaeformis, suggesting high mortality of the species in its early life stages. Recruitment of P. resedaeformis is dependent on the habitat it settles in. It is thought that more suitable habitats with hard substratum, high water velocity, and high food availability have higher recruitment rates.

Environmental importance
Species of Primnoa are ecologically important as they act as keystone species, foundation species, and ecological engineers. P. resedaeformis is a foundation species for deep-sea benthic ecosystems, as it protects from currents and predators, creating suitable habitats for deep-sea benthic species, especially during juvenile stages. The environmental importance of P. resedaeformis can also be related to commercial importance. The deep-sea ecosystem makes for productive fishing grounds, so the decreased abundance of soft corals impacts the fishery as they provide refuge for larval and juvenile stages of economically-important species. Deep-sea corals are not a current resource, but there is growing potential to use these corals as nutritional supplements, enzyme sources, and more.